Inspection of material condition is an important aspect of cost effective maintenance of high value assets (such as aircraft, trains, and other vehicles; transportation infrastructure; refineries, pipelines, other oil and gas infrastructure, to name a few). Major factors driving inspection costs include the cost of the equipment, the amount of time it takes to perform the inspection, the amount of disassembly required to perform the inspection, the cost of reassembly (or repair if the inspection is destructive), and the expertise and number of required operators.
Defects of interest vary by application, and include cracks, fatigue, corrosion, stress corrosion crack colonies, inclusions, pits, dents, gauges, corrosion-fatigue, cracks in dents, and other combinations of defects and other defects caused by service, manufacturing, or other events and processes.
A variety of sensor technologies have been developed to support the inspection needs of industry. Electromagnetic methods for inspection include Radiography, eddy-current testing (ET), Magnetic Flux Leakage (MFL), Magnetic Particle Testing (MPT or MT), Electromagnetic Acoustic Transmission (EMAT) and other variations on these and other methods.
In general, for advanced ET methods transimpedance is measured as indicated in FIG. 3. A signal generator 112 creates a sinusoidal waveform signal. This signal is applied to the system being tested, in this example, sensor 120. Multiplier 114-A multiplies the output of sensor 120 with the original signal and the result is passed through a low pass filter (LPF) 114-B to eliminate all frequency components except zero. The output of the filter is the real component of the transimpedance. To obtain the imaginary (90° phase) component, the reference signal used in the multiplication is shifted by 90°.
Multiplication and low-pass filtering is accomplished with electronics operating on the analog signal output from signal generator 112 and sensor 120. The output of LPF 114-B may be converted by an analog to digital converter for later processing or presentation on a digital display. There is a certain length of time that needs to pass between the time the signal is applied and a valid measurement can be taken, due to settling time of LPF 114-B.